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We identify the function of CuO impurities in CuBi2O4 photocathodes significantly improving the photoelectrochemical water reduction efficiency. Spectroelectrochemical photo/voltage-induced absorption spectroscopies are employed to investigate the charge carrier dynamics of pure, CuO-impure, and CuO-covered CuBi2O4 photocathodes during photoelectrochemical water reduction, in comparison with the CuO/FTO for electrochemical water reduction. Electrons after photoexciation are found to accumulate at the surface CuO in both CuO-covered and impure CuBi2O4 photocathodes, rather than directly reducing water from the CuBi2O4 conduction band. A rate law analysis is employed to compare the kinetics of water reduction in CuO/FTO electrochemically and CuBi2O4 photoelectrochemically, showing a faster water reduction rate constant in CuO than CuBi2O4. The kinetic comparison demonstrates that the CuO impurity functions as a cocatalyst on the surface of CuBi2O4 during water reduction due to the faster kinetics. Our results demonstrate the importance of considering the catalytic function of surface impurities responsible for the enhancement of water reduction efficiency.
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